Btrfs: fix shrinking truncate when the no_holes feature is enabled

If the no_holes feature is enabled, we attempt to shrink a file to a size
that ends up in the middle of a hole and we don't have any file extent
items in the fs/subvol tree that go beyond the new file size (or any
ordered extents that will insert such file extent items), we end up not
updating the inode's disk_i_size, we only update the inode's i_size.

This means that after unmounting and mounting the filesystem, or after
the inode is evicted and reloaded, its i_size ends up being incorrect
(an inode's i_size is set to the disk_i_size field when an inode is
loaded). This happens when btrfs_truncate_inode_items() doesn't find
any file extent items to drop - in this case it never makes a call to
btrfs_ordered_update_i_size() in order to update the inode's disk_i_size.

Example reproducer:

  $ mkfs.btrfs -O no-holes -f /dev/sdd
  $ mount /dev/sdd /mnt

  # Create our test file with some data and durably persist it.
  $ xfs_io -f -c "pwrite -S 0xaa 0 128K" /mnt/foo
  $ sync

  # Append some data to the file, increasing its size, and leave a hole
  # between the old size and the start offset if the following write. So
  # our file gets a hole in the range [128Kb, 256Kb[.
  $ xfs_io -c "truncate 160K" /mnt/foo

  # We expect to see our file with a size of 160Kb, with the first 128Kb
  # of data all having the value 0xaa and the remaining 32Kb of data all
  # having the value 0x00.
  $ od -t x1 /mnt/foo
  0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
  *
  0400000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  *
  0500000

  # Now cleanly unmount and mount again the filesystem.
  $ umount /mnt
  $ mount /dev/sdd /mnt

  # We expect to get the same result as before, a file with a size of
  # 160Kb, with the first 128Kb of data all having the value 0xaa and the
  # remaining 32Kb of data all having the value 0x00.
  $ od -t x1 /mnt/foo
  0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
  *
  0400000

In the example above the file size/data do not match what they were before
the remount.

Fix this by always calling btrfs_ordered_update_i_size() with a size
matching the size the file was truncated to if btrfs_truncate_inode_items()
is not called for a log tree and no file extent items were dropped. This
ensures the same behaviour as when the no_holes feature is not enabled.

A test case for fstests follows soon.
Signed-off-by: Filipe Manana <fdmanana@suse.com>

fs/btrfs/inode.c

@@ -4209,7 +4209,7 @@ int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
 	u64 extent_num_bytes = 0;
 	u64 extent_offset = 0;
 	u64 item_end = 0;
-	u64 last_size = (u64)-1;
+	u64 last_size = new_size;
 	u32 found_type = (u8)-1;
 	int found_extent;
 	int del_item;
@@ -4493,8 +4493,7 @@ int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
 			btrfs_abort_transaction(trans, root, ret);
 	}
 error:
-	if (last_size != (u64)-1 &&
-	    root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID)
+	if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID)
 		btrfs_ordered_update_i_size(inode, last_size, NULL);
 
 	btrfs_free_path(path);